Concrete slabs, in the form of square or oblong tiles, are generally produced by means of a process comprising the following stages: a) mixing of the cement and aggregates with water until a firm consistency is obtained; b) placing of the mixture inside a cube- or prism-shaped formwork (more than 1-meter thick) so as to produce a compact block; c) vibration of the concrete mixture placed inside the formwork, with removal of the air pockets trapped inside also using vacuum techniques; d) allowing the concrete block to set inside the formwork at ambient temperature, for about 1 day, and then removal of the hardened block from the formwork; e) wet-curing of the hardened block for at least 1 month at room temperature, so as to increase the compression strength to a level of about 50 MPa; f) cutting of the concrete blocks into slabs of 10 to 30 mm thickness; g) polishing of the upper surface of the concrete slabs so as to produce an effect similar to that of enamelled ceramic tiles.
The concrete tiles produced using the abovementioned process may be laid on a rough floor using conventional setting adhesives consisting of cementitious mortar, bicomponent organic polymers or other special glues available on the market. Generally, cementitious mortars are more economical, safer and easier to use than adhesives consisting of non-aqueous polymers. However, when a cementitious mortar is used as an adhesive material between the floor and the concrete tiles, there is an immediate differential swelling due to the presence of the mixing water in the fresh cementitious mortar. In reality, this water is rapidly absorbed by the porous base of the concrete, resulting in a dimensional increase in the base of the concrete tile in contact with the layer of fresh setting cementitious mortar, while no significant dimensional change occurs in the top part of the slab exposed to the surrounding environment. Differential swelling results in partial separation of the base of the concrete tile from the layer of setting cementitious mortar on the floor. In other words, the bottom part of the concrete tile, in contact with the water of the fresh cementitious mortar, tends to increase in dimensions compared to the upper surface. The differential change in dimensions causes convex flexure of the side of the concrete tile exposed to the air, the central part of which rises up higher than the peripheral part.
The drawback of separation occurs when the thickness of the concrete tile is reduced--for example from 30 to 10 mm--and/or when the relative humidity of the environment drops--for example from 70% to 50%--since, in these cases, the swelling which occurs on the base of the concrete tile in contact with the water is accompanied and aggravated by shrinkage of the upper surface exposed to the air.